1. Technical Field of the Invention
This invention relates generally to mixed signal circuitry and more particularly to digital to analog converters.
2. Description of Related Art
In general, a system on a chip (SOC) integrates multiple independent circuits, which are typically available as individual integrated circuits, on to a single integrated circuit. For example, a multimedia (e.g., audio, video, graphics, and/or text) processing SOC combines a processing core (e.g., microprocessor and/or digital signal processor, instruction cache, and data cache), an audio codec (e.g., digitization of analog audio input signals and converting digitized audio signals into analog output signals), a high speed serial interface (e.g., universal serial bus (USB) interface), a display interface, a video decoder, video digital to analog converters (DACs), and an external memory interface.
In a multimedia SOC, the video decoder outputs a digital video signal in a component, composite, or S-video format. One or more of the video DACs converts the digital video signal into an analog video signal, which is provided to one or more off-chip video output jacks. Typically, one or more 75 Ohm cables are coupled to the one or more off-chip video output jacks to provide the analog video signal to an analog video display, which has a load impedance of 75 Ohms. To provide impedance matching, the output of each video DAC is terminated with a 75 Ohm load, which may be on chip or off chip. This, however, requires sufficient power to drive the 75 Ohm load and the 75 Ohm termination.
To reduce the power requirements, short video cables (e.g., less than 0.5 meter) may be used such that the 75 Ohm termination may be omitted. While this reduces the power requirements for the video output, it does so at the cost of requiring short video cables, which may be impractical for many uses of the SOC.
Since the analog video signal may be outputted on one or more video output jacks, it is desirous to only activate the video DACs that are connected to a cable. As such, many multimedia SOCs include jack sense circuitry to detect when a cable is coupled to a particular jack. A known embodiment of a jack sense circuit includes a pull up resistor, a mechanical switch, and use of a general purpose input/output (GPIO) pin. When a cable is plugged into a jack, the mechanical switch is closed, coupling the pull up resistor to ground. The GPIO pin is coupled to the common node of the mechanical switch and the pull up resistor, which, in this state, is coupled to ground via the closed mechanical switch. When a cable is not connected to the jack, the mechanical switch is open and the pull up resistor is floating. The GPIO pin is pulled up to Vdd via the pull-up resistor to indicate that no cable is connected to the jack.
While the above described jack sense circuit works to detect whether a cable is plugged into a jack, it requires the use of a GPIO pin and a pull up resistor for each of the video DAC outputs. In addition, the jack sense circuit only detects whether a cable is plugged into a jack; it does not detect whether other end is coupled to a load, or whether the load is a video display or headphones. As such, if a floating cable is connected to a jack, the jack sense circuit provides a positive indication, which is used to enable the corresponding video DAC. This wastes power since the enable video DAC is not being used since the cable is not connected to a video display.
Therefore a need exists for a multimedia SOC that includes an improved jack sense circuit and/or includes a more efficient video DAC.